Trichomoniasis is one of the three major causes of vaginal infection, along with yeast infections and bacterial vaginosis (BV). It's the only one of the trifecta, though, that is officially considered an STI. And yet, it's probably the one you have never heard about before.

According to the CDC, trichomoniasis (often referred to as trich), is the most common curable STD. An estimated 3.7 million people in the U.S. have it, but only about 30 percent develop symptoms. Men are more likely to be asymptomatic, but about half of women may have this infection without realizing it, Michael Cackovic, M.D., an ob/gyn at Ohio State Wexner Medical Center, tells SELF.

For those who do experience symptoms, they're not pleasant. "The biggest symptoms people will come [to me] with is abnormal discharge, pain, irritation and sometimes bleeding a little after intercourse," Cackovic says. Trich can give discharge a bit of a green tinge, but it's the smell that makes the infection the most identifiable. "Trich has a very distinct, putrid odor," he explains. "It’s so distinct [that an ob/gyn] can sometimes notice it before they even put the speculum in."

So what exactly is trichomoniasis, and who's most at risk for getting it? Here are the basics you need to know.

Trichomoniasis is caused by a parasite that's passed from one partner to another during sex.

The parasite is called Trichomonas vaginalis, a type of single-celled organism (or protozoa). ** "It’s alive, and when you look under the microscope you can see it swimming around," Cackovic explains. During intercourse, these parasites can easily travel from one person to another—from vagina to penis, penis to vagina, and vagina to vagina. Sperm doesn't need to be involved. They like to colonize in squamous epithelium cells, which are found in the vagina, Cackovic explains, but they can also move up into the urethra. In men, that's the only place where trich parasites will congregate.

It's more common in women than men, and often happens alongside bacterial vaginosis.

Trich affects more than 2 million women ages 14 to 49 in the U.S. Like UTIs (and so many other health problems) female anatomy makes you more susceptible. Trich parasites need somewhere to colonize, Cackovic explains, and it's tougher for them to stay in a man's urethra long term (which is being flushed often) versus a woman's vagina. It's simply a cozier place for parasites to hangout.

Oftentimes, trich and BV occur together. Cackovic says he sees this often. One explanation is that the changes in the vagina from one infection increases the risk for developing the other. "BV makes vaginal discharge alkaline as opposed to acidic, so it makes [the environment] better for trich to live there." A normal acidic vagina will be better at protecting itself from intruders.

Many women don't experience symptoms, but those who do notice abnormal, foul-smelling discharge, and pain, burning, or irritation during sex or urination.

Even if you don't experience symptoms when you first get it, chances are, if trich goes untreated and gets bad enough, you may start to have symptoms eventually. "But some women just aren’t attuned to what's going on down there," Cacokovic says, which can cause some to blow off minor symptoms and not see a doctor until things get super irritated and uncomfortable–or ever. That's why it's important to see your ob/gyn if you notice any change in your discharge color and smell, or experience vaginal pain or burning.

There's a lot we don't know about trichomoniasis—and not a lot being done to stem its spread.

Even though trich is extremely common, it's not a reportable disease, which means that local and state health departments and the CDC don't require new cases to be reported when they're diagnosed. This also means that public health programs aren't really putting effort into tracking or controlling its transmission. Because of this, "we don’t have a lot of data on it we don’t know a lot about it," Cackovic explains. "We know it’s really prevalent, but we don’t know just how prevalent it is." There's also no sure answer on the incubation period (aka, how long after exposure symptoms begin to show)—it's estimated to be anywhere from five to 28 days. "My guess is that because it doesn’t harbor the same potential long-term consequences as other STIs, like PID [which can cause infertility], the thought is probably that it’s just not worth what it would cost," to track and monitor it, Cackovic suggests.

Having trich may increase your chances of getting another STD, and can cause pregnancy complications.

Unlike chlamydia and gonorrhea, which can lead to infertility if left untreated, trich can't travel up your reproductive tract and wreck havoc on those organs. The parasites will stay localized, and may continue to multiply but not spread. However, the infection can cause some complications for pregnant women. Trichomoniasis is linked to preterm delivery and low birth weights. "In two separate occasions in my career, I've seen it colonized in amniotic fluid," Cackovic says, though there's really no recorded statistics on this.

The good news is that trich is easily treatable with one swift round of antibiotics.

The same oral antibiotics given for BV—metronidazole or tinidazole—are used to treat trich. Symptoms should start to ease up a day or two after starting, and can take anywhere from five to 10 days to clear up completely. If one partner learns they have trich, both need to be treated, regardless of symptoms, to avoid unknowingly passing it back or along to someone else. "It's also recommended to check back in with your doctor two weeks after treatment to makes sure it’s cleared up," Cackovic says. For some men, trichomoniasis clears up on its own without medication, he adds. But most of the time, and typically for women, trich can last for months or years if left untreated.